Update README and CHANGELOG overview features list

doc/user_guide.md

@@ -15,6 +15,7 @@ Propane is a LALR Parser Generator (LPG) which:
   * generates a table-driven shift/reduce parser to parse input in linear time
   * targets C, C++, or D language outputs
   * optionally supports automatic full parse tree generation
+  * supports starting parsing from multiple start rules
   * tracks input text start and end positions for all matched tokens/rules
   * is MIT-licensed
   * is distributable as a standalone Ruby script